This invention relates in general to the manufacture of cement clinker, particularly in rotary kilns. More specifically, the invention relates to the introduction of slag into the kiln to enhance the clinker production process.
The details of a typical cement pyroprocessing operation are well known. For purposes of illustration, one type of rotary kiln for manufacturing Portland cement is depicted in FIG. 1. Air and a primary fuel, such as coal, are injected into the firing or clinkering end of the kiln and are combusted to supply heat energy. Wet or dry raw materials, known as raw mix, for producing cement, such as limestone, clay and sand, are injected into the feed end of the kiln opposite the clinkering end. The kiln is inclined so that as the kiln rotates, the raw materials move through the kiln counter-current to the direction of the flow of the hot combustion gases so that the raw materials are subjected to progressively higher temperatures. For instance, at the input end, a pre-calcining zone can be provided that has a gas temperature of about 1000° F. (538° C.). The kiln gas temperature can be increased to about 1600° F. (871° C.) in a calcining zone where the CaCO3 in the raw materials is decomposed. The calcined material then passes to a clinkering zone where it faces the burning zone temperature inside the kiln, approximately 2732° F. (1500° C.). It is in this zone that the feedstock is converted into the typical cement compounds, such as tricalcium silicate, dicalcium silicate, tricalcium aluminate, etc. A cooling zone follows at the output of the kiln. The resulting compound, or clinker, is later mixed with other materials, such as gypsum, and then finely ground to produce Portland cement.
A variety of materials have been added to the cementious compositions in the production of cement clinker to help increase the clinker yield. The use of slag in the production of cement dates back to 1774 when a mortar was made with ground blast-furnace slag and slaked lime. The first commercial use of blended slag cements arose in Europe in the mid-1800's. A current example of a blended cement using a blast-furnace slag is found in U.S. Pat. No. 5,976,243. Many slags are well suited for use in clinker production because the slags can include many chemical constituents common to the cement chemical compounds. In addition, many slags can be added to the cement clinker without any deleterious effects to the cement kiln or to the clinker product. Thus, the slag, which is essentially a waste material from metal production, takes on value and can reduce the quantity of more expensive feedstock or virgin feedstock required in the clinker production.
The use of slag as a feedstock in the pyroprocessing of cement, including Portland cement, is also known. For example, U.S. Pat. No. 2,600,515, issued in 1952, discloses introducing blast-furnace slag directly into the flame in order to avoid problems associated with the fusibility of slag. The patent of Young, U.S. Pat. No. 5,421,880, describes introducing a steel slag at the feed end of the kiln to combine with the feedstock material, and then heating the mixture to form cement clinkers. This patent, and a subsequent U.S. Pat. No. 5,494,515 to the same inventor, contemplate adding a crushed and screened air-cooled slag to the lime-containing feedstock material.
It can be advantageous to burn waste materials in cement kilns, for several reasons. Such wastes would otherwise have to be disposed in a landfill or other long term containment, or incinerated as a means of destroying the materials. Landfill disposal typically is more expensive and less desirable than disposal by recovering the useful energy value of the waste. While these wastes provide energy to the kiln system, the kiln operator typically charges a “tipping fee”, or service charge for accepting and disposing of the waste. The tipping fee is charged because there usually is a cost for handling and/or for pollution control associated with the use of diverse waste streams. Thus, use of waste-derived fuel in a cement kiln provides a benefit to the fuel user and to the waste generator. Namely, the kiln operator may gain significant income from tipping fees as well as fuel value that reduces the demand for conventional fossil fuels, and the waste generator may have access to a lower cost disposal option for the waste. The environment also benefits from use of waste as fuel, because cement kilns have efficient destructive capacity for various wastes as fuel and resultant fuel combustion products, due to high burning zone temperatures and long retention times of materials in the high temperature zone. Valuable landfill space is conserved, fossil fuels are conserved, and wastes that might have contaminated land or water are efficiently destroyed.
Types of waste that have been used as fuel or that have been recycled or processed in a variety of high temperature kiln situations, including cement kilns, according to the prior art include waste tires, either whole or when reduced in size by some means (U.S. Pat. No. 5,473,998); hazardous waste liquids, or solids or both (U.S. Pat. No. 5,454,333); agricultural waste, for example rice hulls; paper mill sludge (U.S. Pat. No. 5,392,721); soil, sludge, sand, rock or water contaminated with organic solvents and/or toxic metals (U.S. Pat. No. 4,921,538); sewage sludge (U.S. Pat. No. 5,217,624); petroleum refinery sludge (U.S. Pat. No. 5,141,526); various hazardous combustible wastes (see U.S. Pat. No. 5,454,333 or U.S. Pat. No. 4,984,983) and non-hazardous low-grade fuel wastes such as wood, paper and chemical waste (U.S. Pat. No. 5,336,317).
Waste that has little or no fuel value, that is, principally inorganic waste, also may be added to cement kiln raw material mix. Cement kilns consume a large quantity of raw materials consisting principally of calcium carbonate (CaCO3), silica (SiO2), alumina (Al2O3), and iron oxide (Fe2OP3). Raw materials also include lesser amounts of other compounds including magnesium carbonate or oxide, sulfur oxide (usually SO3 or SO4), and compounds of titanium, phosphorous, potassium, sodium, chloride, and others. Some of these may be desirable and some undesirable or detrimental in the raw material mix used for making cement. Each should be present as a set percentage of the whole or have an upper limit on the allowable percentage in the raw material mix that will be determined by the individual properties of each kiln system and the chemistry of its raw material supply. Industrial and other wastes that contain significant amounts of one or more of these compounds may be used to augment raw material for a cement kiln.
The usual locations for the input of fuel, air and raw mix are at the opposite ends of the kiln. In addition, flue gases escape at the elevated feed end of the inclined rotary kiln tube. Waste-derived fuel is sometimes added as a supplemental fuel at the mid-kiln location shown in FIG. 1. The temperature at this mid-kiln location is high enough to assure complete substantially complete combustion of the waste so that the kiln can derive fuel value from the waste material. Waste type fuels that have been introduced at a mid-kiln location include hazardous waste materials, as disclosed in U.S. Pat. No. 6,050,203, and whole or shredded tires, as disclosed in U.S. Pat. No. 6,213,764.
An important problem that must be addressed in clinker manufacturing is the emission of pollutants in the flue gases of cement kilns fueled according to the prior art. Flue gas exiting a cement manufacturing plant will contain products of combustion, including carbon dioxide and NOx, along with water vapor and other gases from drying and chemically altering the raw materials in the kiln. The major fuels used in cement making are coal, oil, natural gas, and wastes, where coal is the principal fuel in the industry worldwide. Since coal is predominantly carbon, which oxidizes during combustion to form carbon dioxide, the flue gases from coal fired kilns contain a substantial proportion of carbon dioxide as a combustion product. Moreover, nitrogen oxides, NOx, including at least nitrogen monoxide, NO, and nitrogen dioxide, NO2, are formed from atmospheric oxygen, atmospheric nitrogen and the compounds containing nitrogen in the fuel. At combustion temperatures above about 1600° C., with sufficiently long dwell times of the combustion gases in the flame, the essential product formed from the molecular nitrogen can be nitrogen monoxide, NO. Significant effort has gone into eliminating NOx emissions, such modified fuels and scrubber system.
There is a consistent need to improve the efficiency and yield of the cement clinker production process. Moreover, the need remains a constant requirement to reduce pollution causing discharges from the cement kilns. There is also a common over-riding need to reduce pollutants of all types, including solid waste materials.